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Question
Describe, with the help of a circuit diagram, the working of a photodiode.
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Solution
Photo diode is a p-n junction diode made of photosensitive semiconductor in which current carriers are generated by photons through photo excitation, i.e., photo conduction by light. It is operated under reverse bias below the breakdown voltage.
Working of Photo diode
- The diode is fabricated such that the generation of e-h pairs takes place in or near the depletion region of the diode.
When visible light of energy greater than forbidden energy gap ( \[h\nu > E_g\] ) is incident on a reverse bias p-n junction photo diode, additional electron-hole pairs are created in the depletion region ( or near the Junction).
- These charge carriers will be separated by the junction and made to flow across the junction, creating a reverse current across the junction.
- The direction of the electric field is such that electrons reach n-side and holes are
- The value of reverse saturation current increases with the increase in the intensity of incident light as shown here. Hence, thereverse saturation current through the photo diode varies linearly with the light intensity.
- collected on p-side giving rise to an emf.
I – V characteristics of photo diode
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